1. Field of the Invention
The present invention relates to the purification of natural gas, and in particular to glycol regeneration systems for the removal of undesirable organic compounds from glycol used in the dehydration of natural gas.
2. Description of the Prior Art
Natural gas obtained from wells typically contains water vapor along with other hydrocarbons which are considered impurities of the natural gas. These impurities, particularly water, can cause many problems in pipeline and processing equipment. For this reason, it has long been a common practice to treat the natural gas near or at the well head to remove these impurities. Removing water vapor and other hydrocarbons helps to prevent clogging of the pipeline with liquid water or hydrocarbons, and also helps prevent corrosion of the pipelines.
The most commonly used method of removing water from the natural gas is through glycol dehydration. The glycol is expensive, so it is advantageous to recycle (regenerate) it for further use. Removing the water in the glycol typically entails heating the glycol to between 350.degree. F. and 400.degree. F. at atmospheric pressure. This vaporizes the water to leave a pure stream of glycol for reuse. However, other hydrocarbon impurities are also removed from the glycol, and simply heating the glycol in a traditional atmospheric pressure reboiler also vaporizes those hydrocarbons, which are thus released into the atmosphere. This presents an environmental problem.
Recent developments in environmental regulations reflect a concern for the detrimental effects of aromatic hydrocarbon emissions on the atmosphere. This increased concern has led the gas industry to look for new ways to reduce and control hydrocarbon emissions, including emissions of benzene, toluene, elthylbenzene, and xylene ("BTEX" ), from glycol dehydrators. The EPA estimates that there are approximately 40,000 glycol dehydration units in the U.S. that account for about 88% of the total BTEX emissions for the oil and gas industry. The control of BTEX emissions is one of the largest environmental challenges facing the U.S. natural gas industry.
The typical natural gas glycol dehydration process transfers water from the gas to a liquid stream of glycol and simultaneously removes a small portion of the BTEX and other volatile organic compounds. During the regeneration of the glycol, the water (in the form of steam) and the absorbed hydrocarbons are emitted to the atmosphere or further processed through an emissions control technology.
Of the industry producers that are employing BTEX control technology, about 80% use a vapor recovery system ("VRS" ) to control emissions. The other 20% employ flare or incineration systems to destroy all organic compounds vented from the glycol purifier. A VRS consists of the condensation of water and heavy hydrocarbons using either air, water, or glycol as the cooling agent, followed by three phase separation. Most of the systems employ an atmospheric reboiler, where the pressure within the system is reduced to atmospheric pressure and the rich glycol is heated to 350.degree. F. to 400.degree. F., and the resulting gas is condensed to remove the water and BTEX. The products of separation are condensable hydrocarbons, water in its liquid form, and non-condensable gases. Because separation occurs at atmospheric pressure, all three phases must be pumped or compressed to storage tanks and the gas system. The incineration systems have high operating costs and do not recover any hydrocarbons for sales.
Often, some of the non-condensable hydrocarbon gasses in the glycol stream are released while still under pressure--before being boiled off under heat and at atmospheric pressure. This is often accomplished in the flash tank. However, the glycol is not heated, or heated only slightly. Given that the boiling points of compounds will increase while under pressure, the temperature is not enough to remove BTEX, nor is any attempt made to remove BTEX gasses at the flash tank.
An alternate method of removing BTEX from glycol is a process called R-BTEX. The R-BTEX system and all VRS systems involve the condensing of the water stream coming from the dehydration unit and then separating the components at atmospheric pressure. This methodology has two significant flaws. One is the expense of condensing the steam into it liquid state. The other is the problem of pressurizing the separated components for transfer to their appropriate locations.
More recent methods of separating BTEX and non-condensable hydrocarbons from glycol is by the use of a stripping gas with gas bubble-caps located within a stripper vessel under pressure. (U.S. Pat. No. 5,536,303). Problems with this system include the initial cost of the complex array of bubble caps themselves, the inability of the system to recover and thus use the BTEX and other components that are carried off by the stripping gas, and the stripping gas required is often more than can be used as fuel, the excess being flared which adds to waste of material. Thus, there is a need for an improved means of removing BTEX compounds from glycol used in natural gas production.